The present invention relates to a pressure sensor assembly for sensing gas pressure, and may be more particularly applicable to a sensor assembly for sensing exhaust gas recirculation (EGR) backpressure.
For certain systems that employ gas to function, it is important to accurately measure the gas pressure at a particular point in the system. In such systems, then, a sensor is employed that measures the gas pressure. Such systems may be employed in such technology areas as automotive, industrial, aerospace and process controls.
For example, some turbocharged, direct injected engines equipped with variable cam technology have demonstrated considerable improvement in fuel economy by the addition of cooled external exhaust gas recirculation (EGR). The exhaust gas can be drawn from the exhaust stream either post turbine for a low pressure EGR application or pre-turbine for a high pressure EGR application. With a high pressure EGR application, in order to assure sufficient accuracy of an EGR mass flow estimation through an EGR valve to an intake manifold having an active wastegate system, it is important to accurately detect the absolute backpressure in the EGR gas.
One current methodology employed on high pressure diesel engine EGR applications uses a ceramic capacitive technology. This type of sensing mechanism produces an increase in capacitance proportional to a corresponding increase in EGR gas pressure. Signal conditioning electronics then provide voltage output values that vary according to variations in pressure to an electronic control module. The electronic control module then uses this EGR pressure information in its engine control strategy. While EGR gas measurements are obtained using this method, it has drawbacks. First, this sensor technology may be too bulky for an EGR valve sensor assembly—that is, the assembly may not package appropriately in particular vehicles. Second, this type of pressure sensor assembly may be more expensive than is desirable for use in particular vehicle and other types of systems.
Another methodology employs a silicon piezoresistive Wheatstone bridge pressure sensing technology. This methodology reduces the packaging size and cost versus ceramic capacitive technology. However, with this silicon technology, the sensor mount may be subject to adhesive bond joint fatigue/failure resulting from the relatively high exhaust gas backpressure combined with exhaust pulsating pressure amplitude. Adhesives securing silicon piezoresistive sense elements do not generally provide a reliable long term bond, especially for gas pressures above two bar absolute and 125 degrees Celsius, and when combined with pressure pulsations inherent with exhaust gas pressures from an internal combustion engine.